Pump-probe interferometry

Things are not quite as simple as they seem. In order for the constructive interference, which is at the core of wavepacket interferometry, to occur, not only must (t + At) = (t), but also the phases of apump and aprobe> which depend on the optical phase of the femtosecond laser rather than the molecular phase, must match. A rigorous treatment of the phase coherent pump/probe scheme using optically phase-locked pulse pairs is presented by Scherer, et al., [1990, 1991, 1992] and refined by Albrecht, et al., (1999), who discuss the distinction between and consequences of pulse envelope delays vs. carrier wave phase shifts (see Fig. 9.6). A simplified treatment, valid only for weak optical pulses is presented here. 

This theorem can be used to determine the correct relative phase factor. The entire spectral interferometry analysis described in this section is performed, changing the relative phase factor used until the projection of the real 2DPE spectrum in energy matches the spectrally resolved pump probe spectrum. In all of the work presented in this chapter, a non-frequency-dependent phase factor... 

One of the most important yet simple ideas that ignited excitement about fem-tochemistry is wavepacket interferometry (Salour and Cohen-Tannoudji, 1977 Scherer, et al., 1990, 1991, 1992 Jonas and Fleming, 1995 Weinacht, et al., 1999), an optical form of Ramsey-fringe spectroscopy (Ramsey, 1990). A molecular system is subjected to two identical optical pulses created by splitting one pulse at a beam splitter. The two pulses are called the pump and the probe . The time delay between pump and probe pulses is scanned systematically using an optical delay line. The optical arrangement is very similar to that of a Fourier Transform Spectrometer (Heller, 1990). The difference in the paths traveled by the pump and probe pulses, Ad, before the two pulses are recombined at a second beam splitter corresponds to a time delay, At = Ad/c, where c is the speed of light. 

For 900 nm radiation, the path difference between pump and probe is stepped in 0.9 /j,m increments (or integer multiples), which corresponds to At, = 3 fs. This crucial modification is called phase coherent wavepacket interferometry (Scherer, et al, 1990, 1991, 1992) and it results in a profound simplification of... 

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